Solar receiver, method of cooling a solar receiver and a power generation system

ABSTRACT

A solar receiver ( 100 ), for capturing solar radiation, comprising a radiation capturing element ( 3 ) and a channel ( 8 ) around that element, through which channel ( 8 ) a pressurised working fluid is passed to absorb thermal energy from the radiation capturing element.

TECHNICAL FIELD

The present disclosure relates to solar receivers for capturing solarradiation and converting it into thermal energy of a working fluid. Thedisclosure also relates to a method of cooling a solar receiver and apower generation system comprising such a solar receiver.

BACKGROUND ART

The field of power generation systems using renewable energy sourcescomprises the conversion of energy from the sun's radiation into usefulwork that can then be used to generate power such as electricity. Onemeans by which this conversion might be achieved is through that ofsolar heating of a working fluid such as a liquid or a gas, which fluidonce heated may then be used to drive some form of turbine to generateelectrical power. Systems that operate on this principle may employlarge arrays of parabolic mirrors arranged in a precise manner around asolar receiver to reflect radiation from the sun on to a particular areaof the solar receiver. In this manner a system is arrived at that allowsa far larger amount of the sun's radiation to be directed to the solarreceiver than would otherwise be practicable through enlargement of thesolar receiver or some form of concentrating lens. The key factorssurrounding the solar receivers are those of: efficiency of conversionbetween the energy of the sun's radiation and the useful work generated;cooling issues involving ensuring that the solar receiver is capable ofwithstanding the high temperatures that it is subjected to underfocussed solar radiation; and mechanical robustness of the system in theface of operating environments, such as deserts, which often pose suchissues as dust storms and ranges of temperature.

Two forms of solar receiver are direct solar receivers and indirectsolar receivers. Direct solar receivers allow the solar radiation todirectly pass through a window to a working fluid, which working fluidis conveniently a gas such as air. In this instance the solar radiationacts directly upon the working fluid and causes a consequent rise inthermal energy. In an indirect solar receiver system, the solarradiation is interrupted from reaching the working fluid directly by amaterial of some kind such as a solid surface, typically metallic, andit is this solid surface that is heated by the solar radiation and whichthen exchanges its heat with the working fluid via some form ofthermodynamic transfer.

The indirect solar receivers have been proven to be more robust thandirect solar receivers because they require no transparent materialthrough which the solar radiation must pass in order to reach theworking fluid. Such transparent material may take the form of a quartzwindow or similar, which is capable of withstanding high temperaturesbut which is nonetheless relatively fragile to environmental factorssuch as dust and debris, with small cracks formed thereby propagatingthrough the window as the temperature thereof rises and thereby leadingto a failure of the entire solar receiver system. In contrast, anindirect solar receiver system is advantageous because it avoids anyneed for these relatively fragile elements of the system, albeit at theexpense of reduced rate of transfer of energy from the solar radiationto the working fluid.

Once the working fluid has been suitably heated it may then be passedthrough some form of heat exchanger or combustion system to furtherincrease the temperature of the working fluid for use with anelectricity generation system such as a gas turbine linked to anelectrical generator

The efficiency of the system is a function of the amount of solarradiation entering the solar receiver that is effectively captured andtransferred to the working fluid, followed by the efficiency of thetransfer of that energy into useful work for driving the electricalgenerator. An issue that limits solar receivers from reaching maximumefficiency is that of re-radiation from the surface of the solarreceiver back out into the atmosphere, which energy so re-radiated islost for the purposes of power generation. It is therefore advantageousto provide a system that limits as far as possible a degree ofre-radiation. A further factor in maximising the efficiency of a solarreceiver is to limit the loss of thermal energy from the working fluidinto its surroundings before reaching the power generation sub-system.Where the working fluid is pressurised, it is necessary to provide apressure-tight seal around the channel through which the working fluidflows, and this pressure tight seal is difficult to create in the faceof the significant temperature ranges experienced by the receivercomponents. Damage to the seal will lead to unwanted venting of theworking fluid, which may cause damage to the solar receiver as a wholeand will, at the very least, reduce the efficiency of the heat transferprocess.

The present disclosure is aimed at mitigating these issues to provide asolar receiver system having improved efficiency.

SUMMARY

A first aspect of the disclosure provides a solar receiver, forcapturing solar radiation, comprising a hollow radiation capturingelement, a radiation receiving aperture and a flow channel around theelement, through which channel a pressurised working fluid is passedduring operation of the solar receiver to absorb thermal energy from theradiation capturing element.

Preferably, the channel is filled with a porous material through whichthe working fluid flows, which porous material contacts the radiationcapturing element, and wherein the working fluid absorbs at least aportion of the aforesaid thermal energy via the porous material. Theporous material may be reticulated porous ceramic foam, comprising,e.g., silicon carbide.

Preferably, an inlet to the flow channel is arranged to impinge theworking fluid on the periphery of a front portion of the radiationcapturing element proximate the radiation receiving aperture, wherebyimpingement cooling of the periphery of the front portion of theradiation capturing element by the working fluid reduces re-radiation ofcaptured energy out through the aperture. This impingement cooling alsoreduces thermal stresses caused by absorption of solar radiation by thefront portion of the radiation capturing element proximate the radiationreceiving aperture.

Preferably, the solar receiver also comprises a housing for theradiation capturing element, and the radiation capturing element has anoutwardly extending flange for securing the element to a part of thehousing in a pressure tight manner.

The outwardly extending flange may be secured to the housing part by aclamp, and to facilitate the pressure tight seal a gasket may beprovided between one or both of: a) the flange and the housing; and b)the flange and the clamp.

The or each gasket may comprise a material selected from the groupcomprising graphite, ceramic fibres and nickel-based superalloys, but atthe present time, graphite is preferred.

Preferably, a flow path for the working fluid is arranged to impinge theworking fluid on the periphery of the outwardly extending flange to coolit.

Preferably, the flow path directs the working fluid to create anessentially uniform peripheral cooling effect on the front portion ofthe radiation capturing element, thereby to relieve stresses associatedwith thermal gradients.

It is advantageous for cooling of the outwardly extending flange of theradiation capturing element if the flow path for the working fluidincludes chambers formed in the clamp.

Preferably, the radiation capturing element is formed of a non porousmaterial capable of withstanding temperatures of at least 1000° C.

Preferably, the radiation capturing element is formed of, siliconcarbide, for example, sintered silicon carbide or silicon infiltratedsilicon carbide.

Preferably, the radiation capturing element is a cylinder with a domedend opposite the radiation receiving aperture.

To enable delivery of the working fluid to a turbine or other powerproducing device after the working fluid has been heated in the flowchannel around the radiation capturing element, the flow channel mergesinto a working fluid outlet duct of the solar receiver.

Preferably, the working fluid is air or helium.

The foregoing discloses a solar receiver that is advantageously cooledby a working fluid so as to reduce the level of re-radiation itexperiences from the aperture thereof. In addition, substantiallyuniform peripheral cooling of the radiation capturing element at andnear its radiation receiving aperture reduces thermal stresses thereon,reducing wear and prolonging the lifetime of the system.

A second aspect of this disclosure provides a method of cooling a solarreceiver comprising a radiation capturing element, a radiation receivingaperture and a flow channel around the element, the method comprisingthe step of passing a pressurised working fluid through the flow channelto absorb thermal energy from the radiation capturing element.

Preferably, the method includes the step of transferring heat from theradiation capturing element to the working fluid by passing the workingfluid through a porous material in the flow channel, the porous materialbeing in contact with the radiation capturing element.

Preferably, the method further includes the step of impinging theworking fluid on the periphery of a front portion of the radiationcapturing element proximate the radiation receiving aperture, wherebyimpingement cooling of the periphery of the front portion of theradiation capturing element by the working fluid reduces re-radiation ofcaptured energy out through the aperture.

The method may further include the step of impinging the working fluidon the periphery of an outwardly extending flange of the radiationcapturing element proximate the radiation receiving aperture in order tocool the flange.

Advantageously, the method includes the step of directing the workingfluid to create an essentially uniform cooling effect on the peripheryof the front portion of the radiation capturing element, thereby torelieve stresses associated with thermal gradients.

A third aspect of the present disclosure provides a power generationsystem comprising at least one solar receiver as described above,wherein the or each outlet from the flow channel around the radiationcapturing element is coupled to a subsequent power generating plantcomponent, such as a gas turbine. Alternatively, the subsequent powergenerating plant component may be a combustor for further heating of theworking fluid before the working fluid is passed to a gas turbine.

It should be understood that to obtain high power outputs from a solarpowered power generation plant, several individual solar receivers maybe arranged to feed their working fluid outputs in parallel to asubsequent power generating plant component.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described with reference to theaccompanying drawings, in which:

FIG. 1A is a side view, partially cut away, of selected elements of asolar receiver according to a first aspect of this disclosure;

FIG. 1B is a pictorial perspective view of the solar receiver of FIG. 1on a reduced scale

FIG. 2A is a perspective cut away view of selected elements at the frontof the solar receiver of FIG. 1;

FIG. 2B is fragmentary view of a component in FIG. 2A, showing it from adifferent angle;

FIG. 3 is a perspective cut away view of selected elements of the solarreceiver of FIG. 1; and

FIG. 4 is a perspective view of an axial cross section of the solarreceiver of FIG. 1

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Detailed descriptions of specific embodiments of solar receivers aredisclosed herein. It will be understood that the disclosed embodimentsare merely examples of the way in which certain aspects of thedisclosure can be implemented and do not represent an exhaustive list ofall of the ways in which the solar receivers may be embodied. Indeed, itwill be understood that the solar receiver described herein may beembodied in various and alternative forms. The figures are notnecessarily to scale and some features may be exaggerated or minimisedto show details of particular components. Well-known components,materials or methods are not necessarily described in great detail inorder to avoid obscuring the present disclosure. Any specific structuraland functional details disclosed herein are not to be interpreted aslimiting, but merely as a basis for the claims and as a representativebasis for teaching one skilled in the art to variously employ thedisclosure.

The general features of the solar receiver will now be described, withspecific detail of selected elements following thereafter.

With reference to FIG. 1 and FIG. 4, an aspect of the present disclosurerelates to an indirect solar receiver 100 comprising a hollow radiationcapturing element 3 forming the wall of a cavity C into which solarradiation is received through a radiation entry aperture A. Theradiation capturing element 3 is configured to exchange heat that hasbeen generated in the walls of the capturing element 3 by the solarradiation, with a pressurised working fluid, such as air or helium, thatis passed through a channel 8 formed around an outer surface of theradiation capturing element 3 and filled with a porous heat exchangingmaterial P, as described below. The working fluid is pumped into thechannel proximate the radiation entry aperture A, and flows along theexterior length of the element 3, from which it absorbs at least aportion of the thermal energy thereof, before flowing to an outlet ofthe solar receiver 100 and on to a power generation system.

In use, the solar receiver 100 receives solar radiation that has beenreflected from an array of automatically guided minors that keep thereflected radiation concentrated on the solar receiver. In order toincrease the radiation concentration factor, therefore the heat fluxentering the radiation capturing element 3, and thus thermal efficiency,a secondary concentrator such as a compound parabolic concentrator,termed a CPC (not shown), is located in front of the radiation capturingelement. Hence, although it appears in FIGS. 1 to 4 that the diameter ofradiation receiving aperture A is the same as the diameter of the cavityC, during operation of the solar receiver the diameter of aperture Awill be smaller than the diameter of cavity C, because it will bedefined by an exit aperture of the CPC located directly in front of thecapturing element 3. CPCs require a highly reflective surface andtypically operate at temperatures of 100° C. and below. Water cooling ispreferably employed to maintain the temperature within this nominalrange. After concentration by the CPC, solar flux typically of up to5000 kW/m² enters the cavity C.

The shape of the cavity C is designed to minimize the amount of solarenergy that is lost by re-radiation from the inner surfaces of thecavity through the radiation entry aperture A, as restricted by the CPC.The cavity C preferably is formed as a cylinder that is closed at itsrear end and has the radiation entry aperture A at its front end, frontand rear being defined by reference to the general direction in whichsolar radiation enters the cavity. The closed end of the cavity C isdomed in shape, i.e., convex in the rearward direction, preferablyhemispherical, such that the cavity provides a continuous internalsurface extending from the radiation entry aperture A. The cylindricalform is advantageous in that it aids even absorption of the solarradiation around about any given annular portion of the radiationcapturing element 3. The cylindrical form is further advantageous inthat it helps to minimize tensile stresses due to the pressure load.Similarly, the domed end of the cavity C ensures, as far as possible,even distribution of thermal energy about any given annular portion ofthe radiation capturing element 3. The element 3 is preferably formed ofa non-porous material capable of withstanding suitably high temperaturesof, for example, over 1000° C. Advantageously, SSiC (sintered siliconcarbide) is used, as it capable of withstanding a high degree of thermalstresses, and this aids durability of the cavity when in use, asdescribed below. If made of SSiC, the element 3 may be moulded in onepiece, e.g., by hot-pressing and sintering of SiC powder, or it mayalternatively be formed from two or more components. In particular, ifthe radiation capturing element is made from SiSiC (silicon infiltratedsilicon carbide), it is convenient to construct the element by fusingtogether two components, consisting of a cylindrical main body and thedomed end. The walls of element 3 preferably have a uniform annularthickness in the range of about 3 mm to about 15 mm, depending onoperating pressure and material properties. In principle, thinner wallsgive better efficiency and reduced thermal stresses, but the choice ofthickness is based on a trade off between structural robustness of theelement 3 and speed of transfer of thermal energy therethrough. Forexample, using SSiC, a thickness of about 5-7 mm is estimated to besufficient to contain a pressure of 10 MPa.

The diameter of aperture A, as effectively restricted by the exitaperture of the CPC, is chosen to be sufficiently large to receive adesired quantity of solar radiation into the cavity C between thecylindrical walls of the capturing element 3, but sufficiently small tominimize re-radiation of captured radiation back out of aperture A.However, a small diameter of aperture A may lead to additionaldifficulties in focusing the solar radiation into the cavity C, evenwhile benefiting from reduced re-radiation of solar radiation back outof the aperture A. In general, the dimensions associated with theradiation capturing element 3 and flow channel 8 should be chosen tomaximize the amount of radiation entering the cavity C, while minimizingthe amount of solar energy lost from the cavity, maximizing heattransfer efficiency from the capturing element 3 to the working fluid inflow channel 8, and minimizing parasitic losses in the flow of theworking fluid. For example, our copending patent application referenceT10/035-0_GB of even date with the present patent application, discussesoptimization of aperture and cavity dimension in terms of ratios,whereas our copending patent application reference T10/037-0_GB of evendate with the present patent application, discusses optimizing thegeometrical characteristics of the flow channel 8 to reduce pressurelosses in the flow of working fluid through the flow channel 8. Suitablecavity dimensions in absolute terms depend strongly on the power levelof the receiver, but maximum cavity diameter is restricted by themanufacturing process of the element 3 and the associated loss inrobustness for larger dimensions. For example, for a 100 kW receiver thediameter of the cavity C may be about 300 mm, with a length of 500 mm.

As already mentioned, channel 8 surrounds the exterior surface of theradiation capturing element 3. Channel 8 is filled with a porous heatexchanging material P, preferably in the form of a reticulated porousceramic (RPC) foam, which allows passage of the working fluidtherethrough and which provides a means of heat exchange between theexterior surface of the cavity 3 and the working fluid. It is envisagedthat the working fluid may, for example, be air, which will pass readilythrough the pores of the material P. As shown best in FIG. 4, for easeof manufacture and assembly, the porous material P may comprise a stackof annular blocks with an internal diameter matched to the outerdiameter of the cylindrical radiation capturing element, and adisc-shaped block positioned at the downstream end of the channel 8 andabutting the domed end of the radiation capturing element 3.

Disposed around the channel 8 of porous heat exchanging material is avolume of insulation material 31. This insulator advantageouslyprevents, as far as possible, heat losses from the channel 8 and shouldcomprise a material with low conductivity and low permeability.Preferably, as best shown in FIG. 4, the insulating material fills theremaining volume between the channel 8 and an outer housing 10 of thesolar receiver 100.

In one embodiment, the insulating material is made from Al2O3-SiO2(aluminosilicate) fibers. This is a highly porous material withporosities of 80-95% (porosity is defined as (void volume)/(totalvolume)). The fiber diameter is very small, in the order of 1-10micrometers, which leads to a tortuous path for the working fluid,resulting in a low permeability in the order of 10̂(−10) m̂2. Since thepermeability of insulating material is orders of magnitude lower thanthe permeability of reticulated porous ceramic foam (˜10̂(−7) m̂2) theworking fluid is mainly (>99%) flowing through the porous ceramic foamand not into the insulating material, as the resistance across theporous ceramic foam is lower compared to the resistance across theinsulation.

To further increase the ratio of permeability between the insulation andthe reticulated porous ceramic foam, and hence prevent the working fluidfrom entering the insulation it is possible to make the insulation of adense material, e.g. solid Al2O3 (alumina) at the expense of higherthermal conductivity. To have both advantages, high permeability ratioand low thermal conductivity, it is further possible to use fibrousinsulation with low thermal conductivity and add a layer of denseinsulation with low permeability to separate the fibrous insulation fromthe gas flow. The layer can be a layer of ceramic cement based on hightemperature ceramic materials (e.g., Al2O3, SiO2, ZrO2, . . . ) ordirectly by a thin walled structure of ceramic material made from e.g.,Al2O3 or ZrO2.

The housing 10 further comprises a circular aperture plate 6 and, ifnecessary, a fascia plate 1, both formed of steel, which define anaperture 60 that converges from a larger diameter in the outer surfaceof the fascia plate (if present) to a smaller diameter in the surface ofthe aperture plate 6 adjacent the aperture A of element 3. In use, theaperture plate 6, or, if present, the fascia plate 1, interfaces withthe CPC (compound parabolic concentrator, discussed above but not shownin the Figures), whose exit aperture acts to restrict the effectivediameter of aperture A. The aperture 60 is aligned and sized to allowregistry with the aperture A of the cavity C. A pressure-tight gasket 4seals the joint between aperture plate 6 and an out-turned flange 3 a ofthe radiation capturing element 3.

The fascia plate 1 is only necessary if the solar radiation is notperfectly focussed on the CPC. Hence, if present, it only serves as ashield for radiation spillage, i.e., radiation from the solar mirrorarray that overlaps the mouth of the above-mentioned compound parabolicconcentrator. For this purpose the fascia plate 1 is cooled by coolantcircuits 2 embedded in its front surface, which coolant circuits 2comprise small bore pipes formed from a thermally conductive materialsuch as copper. A coolant fluid, such as water, is pumped through thecoolant circuits 2 to transfer away any heat that builds up in thefascia plate 1 and the underlying aperture plate 6. It is advantageousto transfer this heat away from the aperture plate 6 of the housing toavoid thermal warping thereof.

To facilitate manufacture, and as best seen in FIG. 4, the housing 10 ofthe solar receiver 100 comprises two principle parts, a front housingcomponent 19 and a rear housing component 20, which are bolted togetherat their annular bolting flanges 12 and 21, respectively, with apressure-tight annular sealing gasket 13 between them. Housingcomponents 19 and 20 define between them an interior volume that issized and configured to receive the radiation capturing element 3, it'ssurrounding channel 8, a working fluid exit duct 74, and the volume ofinsulation 31 to minimize thermal losses from the channel 8 and the duct74.

A funnel-shaped duct portion 76 enables the annular flow channel 8around the element 3 to merge into the cylindrical outlet duct 74. Ductportion 76 receives hot working fluid from the RPC foam P in thedownstream end of the channel 8 and outlet duct 74 then conveys it toplant in which its energy can be utilized, such as a gas turbine. Anupstream part of duct 74 is accommodated within housing component 20 anda downstream part of duct 74 is accommodated within a cylindricalextension 22 of housing component 20. Duct 74 and extension 22preferably terminate in an outlet fitting (not shown) for connection toan inlet of a power producing system, such as a gas turbine. Note thatthe transition of the flow channel 8 into the outlet duct 74 shouldpreferably be optimized by maintaining a constant flow area to minimizepressure losses in the flow of working fluid therethrough, as describedin. our copending patent application reference T10/037-0_GB of even datewith the present patent application.

It is contemplated that for convenience of manufacture the housingcomponents 19 and 20 and the extension 22 are each formed from sheetsteel. The extension 22 is preferably secured to the housing portion 20by welding, though any other suitably robust means of securement thatcreates a seal between the components would be suitable.

The solar receiver 100 further comprises a plurality of access pointsthrough which it is possible to insert sensors for monitoring the statusof the solar receiver 100. For example, a first access point 25 mayprovides means to insert a thermocouple into the housing extension 22,for measuring the outlet temperature of the working fluid, whereas asecond access point 26 may allow measurement of the external temperatureof the radiation capturing element 3.

Working fluid is directed into the solar receiver 100 through one ormore, for example three, flow channels 70 (see FIG. 4) provided in thedisc-shaped aperture plate 6, each being fed by an inlet tube 41connectable to a pressurised source of the working fluid. Preferably,each inlet tube 41 is directed radially into, and evenly spaced around,an annular external recess 72 in the edge of the aperture plate 6 of thesolar receiver 100.

The source of pressurised working fluid may take the form of, forexample, a pumping system or a pressurised reservoir. The inlet tubes 41may be secured to the aperture plate 6 by welding or brazing, forexample, to provide for a good pressure tight seal.

As best shown in cross section in FIG. 4, each flow channel 70 is formedby a bore that extends substantially radially through the aperture plate6 and terminates in an aperture 50 best shown in FIGS. 2 and 3. Eachaperture 50 opens into an inner circular recess of the aperture plate 6,which recess forms a chamber 62 defined between aperture plate 6, aclamping ring 7 and the radiation capturing element 3, as described inmore detail below.

In the embodiment illustrated, each aperture 50 is defined in the cornerof the recess formed by the intersection of the cylindrical side wall 64of the recess and its annular end surface 65. In this manner, theportion of the aperture 50 defined in the wall 64, together with theportion of the aperture 50 defined in the end surface 65, provide asuitably large cross sectional area to facilitate sufficient flow ofworking fluid therethrough.

The aperture 60 is formed centrally in the aperture plate 6 and has abevelled edge 66 angled such that the aperture 60 narrows as itapproaches the aperture A of cavity C. At its smallest diameter, theaperture 60 is sized to register with an aperture formed in the gasket4, sandwiched between the aperture plate 6 and the out-turned flange 3 aof the radiation capturing element 3, see FIG. 3.

A plurality of (e.g., 12) blind threaded bores are equally spaced aroundthe aperture plate 6 at a fixed radial offset from the interior wall 64to receive set-screws 56 or the like for securing the clamp 7 to therear of the aperture plate 6, the clamp 7 being provided withcorresponding through bores to receive the shanks of the screws 56. Thelocations of these bores are chosen so as to avoid penetrating the flowchannels 70 that pass radially through the aperture plate 6.

As illustrated particularly in FIGS. 2A, 2B and 3, the clamp 7 isbasically an annulus with an outside diameter D and an inside bore 71 ofdiameter d. However, it has been modified by (a) machining a shortcountersunk bore 68 of diameter d¹ in the front side of the clamp 7, d¹being larger than diameter d, to create an inwardly extending flange 69;and (b) machining a plurality of (e.g., six) equally spaced recesses orchambers 54 into the front face of the flange 69. The chambers 54 aregenerally rectangular or square when seen in plan view and at theirradially inner sides are open to the inside bore 71 of the clamp 7. Asshown in FIGS. 3 and 4, when clamp 7 is secured to the aperture plate 6,the chambers 54 communicate between the chamber 62 and the flow channel8 surrounding the radiation capturing element 3, and flange 69 of clamp7 clamps the out-turned flange 3 a of the radiation capturing element 3against the rear face of the aperture plate, with apertures A and 60 inregistration with each other. The arrangement of the chambers 54 is suchthat each aperture 50 defined in the aperture plate 6 is equidistantfrom the adjacent chambers 54. This equidistant arrangement isadvantageous in that it enables the working fluid flows to be sharedequally between chambers 54. In the illustrated embodiment, for example,the aperture plate 6 of the housing comprises three apertures 50, andthe clamp 7 provides six chambers 54, each aperture being equidistancefrom its two adjacent chambers 54.

The clamp 7 is advantageously formed of a material capable ofwithstanding high temperatures. One such suitable material would beInconel®: an austenitic nickel-chromium-based superalloy. Inconel alloysare particularly useful in high temperature applications as it has amelting point of over 1300° C.

The gasket 4 is provided to ensure a pressure tight seal between thefront surface of flange 3 a of element 3 and the rear surface of theaperture plate 6. The gasket 4 is preferably formed of graphite, becauseof its high temperature resistance and because its high compressibilityenables it to seal at high pressures. Another gasket 5 is disposedbetween a peripheral portion of the flange 3 a of the element 3 and thefront face of the flange 69 of clamp 7. This gasket 5 has the sameexternal diameter as gasket 4, but a larger internal diameter. In theembodiment shown, gasket 5 extends across portions of the chambers 54formed in flange 69 of clamp 7, but without blocking working fluid flowtherethrough because the external diameter of gasket 5 is less than thediameter of bore 68 of clamp 7.

As described in our copending patent application reference T10/035-0_GB,of even date with the present patent application, thermal efficiency ofthe radiation capturing element is optimised when the ratio of thediameter of the aperture A of the capturing element to the diameter ofthe cylindrical walls of the capturing element lies in the range ofabout 0.3 to about 0.7, preferably about 0.4 to about 0.65, or roughly0.5. These ratios can be achieved by letting the CPC define theradiation receiving aperture A of the capturing element, as mentionedabove. As the above ratio of diameters decreases from a value of 1, theintensity of radiation is reduced on the wall of the capturing element 3near the aperture A, thereby decreasing thermal stresses upon thecontacting portions of the radiation capturing element 3 and theaperture plate 6 of the housing, so aiding maintenance of a pressuretight seal therebetween.

The completed assembly of the aperture plate 6, gasket 4, radiationcapturing element 3, gasket 5 and clamp 7 is best shown in FIG. 3. Thisradiation capture assembly is then united with the housing 10 and itsassociated components, so that the aperture plate 6 becomes part of thehousing. This is accomplished by inserting the radiation capturingelement 3 into a complementarily sized bore in the porous material Pforming channel 8, as shown in FIG. 4. Clamp 7 also fits within a recessformed in the front surface of the insulation material 31. Hence, theradiation capture assembly of FIG. 3 completes the front of the housingassembly. To secure the aperture plate 6 to the front housing part 19,setscrews 59 or the like pass through a set (e.g., twelve) ofequi-spaced bores 58 formed in a peripheral flange of the aperture plateand are screwed into corresponding threaded blind bores in a flange 11of the front housing component 19. A further graphite gasket 9 issandwiched between the aperture plate 6 and the flange 11.

FIGS. 3 and 4 show the flow path of the pressurised working fluid fromthe inlet tubes 41 to the outlet duct 74, via the bores 70 in apertureplate 6, chambers 62 and 54, and the porous material P in the channel 8.The working fluid increases in temperature through transfer of heat fromthe structure of the porous material P. This heat transfer cools theporous material, which in turn absorbs heat from the contacting surfaceof the heat capturing element 3. The cooling effect on the element 3 isgreatest proximate the radiation receiving aperture A, where thetemperature difference between the working fluid and the element 3 isgreatest. The working fluid may, for example, be pressurised to about 10MPa, which is a moderate pressure useful for driving a simple gasturbine. At this and higher pressures and temperatures, it becomesdifficult to maintain pressure tight seals between element 3 and theadjacent structure of the solar receiver. Thus, the clamping of flange 3a of element 3 using graphite gaskets in the way disclosed above isadvantageous in that it allows for longitudinal thermal expansion of theelement 3 during use and also allows limited radial thermal expansion offlange 3 a without compromising the seals achieved by gaskets 4 and 5.Limited thermal spreading of flange 3 a as it heats up may befacilitated by applying a known high temperature anti-stick coating tothe graphite gaskets to lower their coefficient of friction. However, itis also important to note that the impingement of the working fluid onthe periphery of flange 3 a, its passage through multiple chambers 54and under the rear surface of flange 3 a, and its subsequent impingementon the front portion of the exterior surface of the radiation capturingelement 3, creates a substantially uniform impingement cooling effect onthe periphery of flange 3 a and on the periphery of the front portion ofelement 3, thereby significantly reducing the temperature of the frontsection of the element 3. This not only reduces thermal and mechanicalstresses in the flange 3 a, but also minimises radiation losses throughaperture A.

It is envisaged that the working fluid, after being heated by itspassage through the porous material in the channel 8, and exiting theradiation receiver 100 through outlet duct 74, will go directly to apower generation system. Hence, it may be fed directly into a gasturbine, or alternatively it may be fed into a combustion system forfurther heating before being passed to the gas turbine. After the gasturbine it may undergo a heat exchange with a second working fluid,preferably water to create steam for subsequent use in a powergeneration subsystem such as a steam turbine. Both power generationsystems then operate in concert to produce power. Having exchanged itsheat with the second working fluid, the first working fluid may, atleast in the case of air, be vented to the atmosphere. Alternatively, ifa more expensive gas, such as helium, is used, it may be passed back,via a pumping system, to the inlet tubes 41 of the solar receiver 100for a further cycle of solar heating.

It can be appreciated that various changes may be made within the scopeof the present disclosure, for example, the size and shape of thevarious elements of the solar receiver may be altered as required, andthe entire solar receiver may be scaled up or down as required.

It is further contemplated that the radiation capturing element may beformed of a different material from SiC, such as a refractory alloy.This would offer increase structural strength, but at the cost of alower heat conductivity and operating temperature, meaning that thesolar receiver would have reduced efficiency compared to a cavity formedfrom SiC.

In an alternative embodiment, it is envisaged that the apertures 50 maybe entirely formed in either the wall of bore 64 or its annular endsurface 65 as required, through corresponding modification of the way inwhich the channels 70 penetrate the aperture plate 6, and, as necessary,alteration of the thickness of the aperture plate 6. At present, weprefer that the channels 70 are oriented substantially radially in theaperture plate 6. It is, however, contemplated that the orientation ofthe channels 70 and the chambers 54 may be altered such that they guidethe flow in a way that produces a vortex-like flow around the front partof the radiation capturing element 3, thereby further increasing thecooling effect.

As previously mentioned, the fascia plate 1 may be omitted where thereis reduced or no chance of radiation spillage. It is furthercontemplated that the aperture plate of the housing may be water-cooledand formed of alumina

Although the above description has focused on the use of graphitegaskets, it would alternatively be possible to fabricate them fromceramic fibres (e.g., alumina Al₂O₃), or a nickel-based superalloy suchas Inconel®.

It is further contemplated that the channel 8 of porous material P mightbe formed directly onto the external surface of the element 3, ratherthan formed separately and disposed in the insulating material 31 beforethe element 3 is then inserted therein.

Helium has been mentioned above as an alternative working fluid, becausehelium has a higher heat transfer coefficient than air at equal volumeflow rates, which results in slightly higher thermal efficiencies forequal pressure drops.

It will be recognized that as used herein, directional references suchas “end”, “side”, “inner”, “outer”, “front” and “rear” do not limit therespective features to such orientation, but merely serve to distinguishthese features from one another.

1. A solar receiver for capturing solar radiation comprising: aradiation capturing element; a radiation receiving aperture; and a flowchannel around the radiation capturing element, through which flowchannel a pressurised working fluid is passed during operation of thesolar receiver to absorb thermal energy from the radiation capturingelement.
 2. The solar receiver according to claim 1, wherein the flowchannel is filled with a porous material that contacts the radiationcapturing element and through which the working fluid flows, with theworking fluid absorbing at least a portion of the thermal energy via theporous material.
 3. The solar receiver according to claim 1, wherein theflow channel is filled with a reticulated porous ceramic foam thatcontacts the radiation capturing element and through which the workingfluid flows, with the working fluid absorbing at least a portion of theaforesaid thermal energy via the reticulated porous ceramic foam.
 4. Thesolar receiver according to claim 1, wherein the flow channel is filledwith silicon carbide that contacts the radiation capturing element andthrough which the working fluid flows, with the working fluid absorbingat least a portion of the thermal energy via the silicon carbide.
 5. Thesolar receiver according to claim 1, wherein an inlet to the flowchannel is arranged to impinge the working fluid on a periphery of afront portion of the radiation capturing element proximate the radiationreceiving aperture for impingement cooling of the periphery of the frontportion of the radiation capturing element by the working fluid toreduce re-radiation of captured energy out through the aperture.
 6. Thesolar receiver according to claim 1 wherein when in use impingementcooling reduces thermal stresses caused by absorption of solar radiationby a front portion of the radiation capturing element proximate theradiation receiving aperture.
 7. The solar receiver according to anyclaim 1, further comprising a housing for the radiation capturingelement, wherein the radiation capturing element has an outwardlyextending flange for securing the radiation capturing element to a partof the housing in a pressure tight manner.
 8. The solar receiveraccording to claim 1, further comprising a housing for the radiationcapturing element, wherein the radiation capturing element has anoutwardly extending flange for securing the radiation capturing elementto a part of the housing in a pressure tight manner and the outwardlyextending flange is secured to the housing by a clamp.
 9. The solarreceiver according to claim 1, further comprising a housing for theradiation capturing element, wherein the radiation capturing element hasan outwardly extending flange for securing the radiation capturingelement to a part of the housing in a pressure tight manner facilitatedby a gasket between one or both of: a) the flange and the housing; andb) the flange and a clamp.
 10. The solar receiver according to claim 1,further comprising a housing for the radiation capturing element,wherein the radiation capturing element has an outwardly extendingflange for securing the radiation capturing element to a part of thehousing in a pressure tight manner facilitated by a gasket comprised ofa material selected from the group consisting of graphite, ceramicfibres and nickel-based superalloys.
 11. The solar receiver according toclaim 1, further comprising a housing for the radiation capturingelement, wherein the radiation capturing element has an outwardlyextending flange for securing the radiation capturing element to a partof the housing in a pressure tight manner wherein a flow path for theworking fluid impinges the working fluid on a periphery of the outwardlyextending flange to cool it.
 12. The solar receiver according to claim1, wherein a flow path of the working fluid creates an essentiallyuniform peripheral cooling effect on a front portion of the radiationcapturing element to relieve stresses associated with thermal gradients.13. The solar receiver according to claim 1, further comprising ahousing for the radiation capturing element, wherein the radiationcapturing element has an outwardly extending flange for securing theradiation capturing element to a part of the housing in a pressure tightmanner and the outwardly extending flange is secured to the housing by aclamp, with a flow path for the working fluid including chambers formedin the clamp.
 14. The solar receiver according to claim 1, wherein theradiation capturing element is formed of a non porous material capableof withstanding temperatures in excess of 1000° C.
 15. The solarreceiver according to claim 1, wherein the radiation capturing elementis formed of silicon carbide.
 16. The solar receiver according to claim1, wherein the radiation capturing element is formed of sintered siliconcarbide or silicon infiltrated silicon carbide.
 17. The solar receiveraccording to claim 1, wherein the radiation capturing element is acylinder with a domed end opposite the radiation receiving aperture. 18.The solar receiver according to claim 1, wherein the flow channel aroundthe radiation capturing element merges into a working fluid outlet ductof the solar receiver.
 19. The solar receiver according to claim 1,wherein the working fluid is air or helium.
 20. A method of cooling asolar receiver comprising: passing a pressurised working fluid through aflow channel to absorb thermal energy from a radiation capturingelement.
 21. The method according to claim 20, further comprisingtransferring heat from the radiation capturing element to the workingfluid by passing the working fluid through a porous material in the flowchannel and in contact with the radiation capturing element.
 22. Themethod according to claim 20, further comprising impinging the workingfluid on a periphery of a front portion of the radiation capturingelement proximate a radiation receiving aperture for impingement coolingof the periphery of the front portion of the radiation capturing elementby the working fluid reducing re-radiation of captured energy outthrough the radiation receiving aperture.
 23. The method according toclaim 20, further comprising impinging the working fluid on a peripheryof an outwardly extending flange of the radiation capturing elementproximate a radiation receiving aperture in order to cool the flange.24. The method of claim 20, further comprising directing the workingfluid to create an essentially uniform cooling effect around a peripheryof a front portion of the radiation capturing element to relievestresses associated with thermal gradients.
 25. A power generationsystem comprising at least one solar receiver according to claim 1,wherein each outlet from the flow channel around the radiation capturingelement is coupled to a subsequent power generating plant component. 26.A power generation system comprising at least one solar receiveraccording to claim 1, wherein each outlet from the flow channel aroundthe radiation capturing element is coupled to a gas turbine.
 27. A powergeneration system comprising at least one solar receiver according toclaim 1, wherein each outlet from the flow channel around the radiationcapturing element is coupled to a combustor for heating of the workingfluid before it is passed to a gas turbine.
 28. A power generationsystem comprising at least one solar receiver according to claim 1,wherein each outlet from the flow channel around the radiation capturingelement is coupled to a subsequent power generating plant component andseveral solar receivers are arranged to feed their working fluid outputsin parallel to the subsequent power generating plant component. 29.(canceled)
 30. (canceled)
 31. (canceled)